Cathodic protection is the most widespread. To provide this protection a circulation of electric current is usually created between the metal work and the solution surrounding it, a circulation such that the direction of the current flows from the solution to the metal, thus promoting the cathodic reaction in the region of the work to be protected, while transferring the oxidation reaction to the counter-electrode, thus forming an anode.
There are also cases in which the work forms an anode and the counter-electrode a cathode, (protection of certain metals, such as aluminium, which can be passivated in the electrolyte, via an "anodic protection" current).
Be that as it may, when a "cathodic" or "anodic" protection is applied to a work in contact with the ground (or another electrolytic medium) the effectiveness of the protection is usually measured by the value of the potential difference between the work and the electrolyte.
In actual fact, this voltage is measured between the metal of the protected work and a so-called "reference" complementary electrode in contact with the electrolyte. In practice, it is preferable to place this reference electrode some distance from the surface presumed to be protected.
The measurement of this voltage therefore includes a term due to the potential gradient in the ground.
Knowledge of the real metal/ground potential requires this undesirable term to be eliminated from the measurements.
A means usually used for this purpose consists in measuring this potential, after momentarily cutting the imposed protective current. A method which numerous practitioners have gone over to consists in cutting the protective current for about 3 seconds, the measurement(s) being made between 2 and 3 seconds after cutoff, and in then re-establishing the protective current for about 30 seconds, before a new cutoff.
It is therefore known nowadays that in order to check in particular the state of immunity as regards corrosion of a buried metal work, measurements must be made of potential of the work relative to the ground, protective current cut off, in order to eliminate from the measurement in particular the ohmic drop due to the f low of the current into the ground. Cancellation of this drop is in practice done in less than about one millisecond at the time of cutoff. It can therefore be considered as practically instantaneous.
When the work goes from the "under imposed protective current" situation to the zero current situation, a modification also occurs in the potential of the work relative to the ground. This modification is considered to be due to electrochemical phenomena occurring essentially at the interface between the metal and the ground.
Knowledge of the potential jump which occurs at the moment of cutting of the protective current makes it possible to know what the actual potential was in the situation of active protection of the work.
Quantitative knowledge of the amplitudes and rates of change immediately after cutoff makes it possible to determine what electrochemical phenomena are involved when the work, on leaving the protective situation, changes to a corrosion situation.